1. Field of the Invention
The present invention relates to a cutting tool made of a cubic boron nitride (cBN) sintered material substrate coated with Al2O3. More particularly, the present invention relates to a cutting tool made of Al2O3-coated cBN-based sintered material having improved wear resistance and breakage resistance.
2. Description of the Related Art
Al2O3 is a material optimum for cutting iron-based materials because of its excellent chemical stability and hardness. However, Al2O3exhibits a poor toughness. Therefore, a cutting tool mainly composed of Al2O3 has a deteriorated stability against tool failure to disadvantage. In order to overcome this difficulty, a cutting tool consisting of a cemented carbide substrate having a relatively excellent toughness coated with Al2O3 has been commercialized.
In recent years, there is a growing need for high speed and efficiency and dry cutting in response to the trends of environment-friendly production. In the conventional tools, however, the cemented carbide substrate deforms excessively plastically at high cutting temperature, resulting in that the coating layer easily peels off or is destroyed.
As a means for solution to the problems, a method for coating a cBN-based sintered material excellent in high temperature hardness with Al2O3 has been proposed in JP-A-59-8679 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). However, since the adhesion between cBN-based sintered material and Al2O3 coating layer is insufficient and the optimization of the crystallinity of Al2O3 is insufficient, a remarkable enhancement of wear resistance and breakage resistance is not exhibited in cutting of hard materials such as hardened steel or high speed and efficiency cutting of steel.
It is an object of the present invention to provide a cutting tool which exhibits an excellent flank wear resistance and crater wear resistance particularly in cutting of high hardness difficult-to-cut ferrous materials or high speed and efficiency cutting of steel.
A cutting tool according to the present invention is coated with one or more Al2O3 layers on at least a part of the surface of a cBN-based sintered material substrate taking part in cutting. The sintered material substrate comprises cBN in an amount of 20% to 99% by volume and Al2O3 having an average crystalline particle diameter of not more than 1 xcexcm in an amount of not less than 1.0% to less than 10% by volume. The Al2O3 layer has a thickness (d) of 0.5 xcexcm to 50 xcexcm . The average crystalline particle diameter (s) of Al2O3 is from 0.01 xcexcm to 4 xcexcm if the thickness (d) of the Al2O3 layer is from 0.5 xcexcm to 25 xcexcm (0.5 xcexcmxe2x89xa6dxe2x89xa625 xcexcm ), and that of Al2O3 is from 0.01 xcexcm to 10 xcexcm if the thickness (d) of the Al2O3 layer is from more than 25 xcexcm to 50 xcexcm (25 xcexcm less than dxe2x89xa650 xcexcm ).
The incorporation of a proper amount of Al2O, in a cBN-based sintered material substrate makes it possible to increase the adhesion of the Al2O3 layer or interlayer made of TiCxNyOZ having an excellent bonding power with Al2O3, thereby enhancing the cutting properties. A particularly preferred content of Al2O3 is form 3.0% to less than 5.0%. The reason why the adhesion of the Al2O3 layer or interlayer can be thus increased is presumably as follows:
(1) Al2O3 constituting the coating layer and TiCxNyOZxe2x80x94undergo nucleation with Al2O3 contained in cBN-based sintered material substrate as a starting point; and.
(2) the incorporation of Al2O3 in cBN-based sintered material substrate causes the residual stress characteristic to cBN-based sintered material substrate to change, thereby relaxing misfit of coating layer to residual stress (thermal stress, internal stress).
The homogeneous incorporation of fine Al2O3 particles having a particle diameter of not more than 1 xcexcm in cBN-based sintered material substrate makes it possible to accelerate the formation of fine homogenous nuclei during the formation of Al2O3 or TiCxNyOZxe2x80x94layer and hence form an Al2O3 layer having an excellent crystallinity and adhesion. If the content of Al2O3 falls below 1.0% by volume, it causes uneven nucleation during the formation of coating layer, thereby exerting an insufficient effect. On the contrary, if the content of Al2O3 exceeds 10% by volume, the mechanical properties inherent to Al2O3 is presumably reflected in the mechanical properties of cBN-based sintered material, thereby drastically deteriorating the breakage resistance of cBN-based sintered material substrate.
The Al2O3 layer is preferably mainly composed of xcex1-Al2O3. The coating of cBN-based sintered material substrate with xcex1-Al2O3 with a good adhesion makes it possible to inhibit wear on relieve face and crater wear and hence drastically prolong the life of tool. The coating of cBN-based sintered material substrate with xcexa-Al2O3 with a good adhesion, too, makes it possible to inhibit crater wear and prolong the life of tool. However, wear on relieve face can be little inhibited.
Further, the Al2O3 layer can be oriented on (012), (104), (110), (113), (024) or (116) plane with an orientation index of not less than 1.0 to form a coating layer excellent in wear resistance and strength. This orientation index can be defined by the following equation. The method for determining orientation index is described also in WO96/15286 (PCT/SE95/01347), etc.
TC(hkl)=I(hkl)/Io(hkl)xc3x97[(l/n)xcexa3{(hkl)/Io(hkl)}]xe2x88x921 where I(hkl): Intensity of (hkl) diffraction ray in XRD;
Io(hkl): Diffraction intensity in ASTM card of XRD; and
n: Number of diffraction rays used in calculation ((hkl) diffraction rays used are (012),(104),(110),(113),(024) and (116))
In the foregoing cutting tool, the Al2O3 layer may be complexed with the TiCxNyOZxe2x80x94layer to form a laminate. Specific examples of the composite structure include (1) structure comprising an interlayer made of TiCxNyOZxe2x80x94formed on the interface of Al2O3 layer with cBN-based sintered material substrate, (2) structure comprising a TiCxNyOZ layer provided interposed between a plurality of Al2O3 layers, and (3) structure comprising a TiCxNyOZxe2x80x94layer provided as an outermost layer.
Referring to the reason why the thickness of the Al2O3 layer is defined to a range of from 0.5 xcexcm to 50 xcexcm , if the thickness of the Al2O3 layer falls below the lower limit, the resulting coating effect is insufficient. On the contrary, if the thickness of the Al2O3 layer exceeds the upper limit, the coating layer is more liable to peeling, chipping or breakage. The thickness of the Al2O3 layer is preferably from about 3 to 40 xcexcm. In particular, if the thickness of the Al2O3 layer is not more than 25 xcexcm and the average crystal particle diameter (s) of Al2O3 is from 0.01 xcexcm to 4 xcexcm, the resulting product is excellent in flank wear resistance. If the thickness of the Al2O3 layer is more than 25 xcexcm and the average crystal particle diameter (s) of Al2O3 is from 0.01 xcexcm to 10 xcexcm. the resulting product is excellent in crater wear resistance. If there are a plurality of Al2O3 layers, the total thickness of these Al2O3 layers is used to see whether the thickness of the Al2O3 layer is not more than 25 xcexcm.
The formation of the foregoing Al2O3 layer or TiCxNyOZxe2x80x94layer can be accomplished by CVD method such as thermal CVD method, plasma CVD method and moderate temperature CVD method or PVD method such as sputtering method and ion plating method.
On the other hand, the sintered material substrate is composed of cBN and a binder phase. If the content of cBN is not less than 20% by volume, the production of a thick binder phase which forms a mechanically weak point can be inhibited. The binder phase is preferably made of at least one of nitride, carbide and boride of metals belonging to the groups 4a, 5a and 6a in the periodic table and mutual solid-solution thereof as a main component besides Al2O3. The binder phase may further contains at least one of Al and Si incorporated therein. For the preparation of the sintered material substrate, a plasma sintering apparatus, hot press, ultrahigh pressure sintering apparatus, etc. may be used.
Since the cutting tool according to the present invention is made of a cBN-based sintered material substrate mainly composed of cBN having a hardness next to diamond, it exhibits an excellent plastic deformation resistance. Further, the coating with xcex1-Al2O3, which is chemically stable, having a controlled structure makes it possible to improve crater resistance without causing chipping or peeling. Accordingly, the cutting tool according to the present invention exhibits a prolonged life in cutting of high hardness materials such as hardened steel or high speed and efficiency cutting of steel, which is impossible with existing tools due to the rise in cutting temperature.